Some graphics cards conflict with the combination of the screen saver and the setting to run only if the user is inactive. In particular, ATI Radeon cards seem to have this problem.

You have three choices:

1) Buy a graphics card that does not have this problem (probably to expensive).
2) Stop using the screen saver, and use blank or default instead.
3) Allow BOINC to process all of the time, even when you are working at the keyboard. MOST computers do not slow down when doing this (there are some exceptions though).BOINC WIKI

The machine question is a dog's body Windows machine that sits next to me at work. I use a Mac mostly, so it is only used the few times I need to use a windows only program so I have it working on SETI all the time.

The gfx card being used is an on-board Intel chipset - CUDA is not supported so I have not been doing any GPU processing.

To address your options;

1. As above, it's a work machine, so I can't control/change what hardware it uses.
2. Fair point and I've done this to this point.
3. As it's a side machine, it is working all the time. I just wanted to see the SETI screensaver doing it's work.

While I've been typing this I have just managed to fix it. I turned down the Hardware Acceleration setting in the display control panel under Troubleshooting and this is allowing it to run the screensaver. It's odd that this is required to run it when it can show the graphics from the BOINC manager just fine.

The machine question is a dog's body Windows machine that sits next to me at work. I use a Mac mostly, so it is only used the few times I need to use a windows only program so I have it working on SETI all the time.

The gfx card being used is an on-board Intel chipset - CUDA is not supported so I have not been doing any GPU processing.

To address your options;

1. As above, it's a work machine, so I can't control/change what hardware it uses.
2. Fair point and I've done this to this point.
3. As it's a side machine, it is working all the time. I just wanted to see the SETI screensaver doing it's work.

While I've been typing this I have just managed to fix it. I turned down the Hardware Acceleration setting in the display control panel under Troubleshooting and this is allowing it to run the screensaver. It's odd that this is required to run it when it can show the graphics from the BOINC manager just fine.

I believe that some of the Intel chipsets were licensed from ATI, however, I am not very certain about that.

It is interesting that turning down the acceleration on the graphics card would affect this.

What was apparently happening was that the graphics board is somehow signaling to the key stroke detector that a key had been pressed. Nobody has been able to figure out how or exactly why this happens.BOINC WIKI

Count your blessings. At least you actually get the display back. My screen saver starts OK but when I use the mouse or spacebar to get my desktop back and all I get is the "BLACK SCREEN OF DEATH." Only way out of it is to turn the power off and then back on to reboot my machine.